Method for managing in-flight refuelling of a fleet of aircraft

ABSTRACT

Many types of vehicle disturb the environment behind them as they proceed. As a result, a delay between two successive vehicles has to be maintained to avoid a situation where following vehicles are adversely affected by the disturbed environment caused by leading vehicles. Previously, sequencing has been carried out on a “first come, first served” basis but this is not satisfactory. A method of sequencing a plurality of vehicles is disclosed, wherein each candidate vehicle in said plurality of candidate vehicles is a candidate to be allocated the next place in a sequence, said method comprising the steps of: (i) receiving information pertaining to one of said candidate vehicles; (ii) calculating a value to be attributed to said candidate vehicle on the basis of said received information and information received from the candidate vehicle most recently allocated a place in said sequence; (iii) repeating steps (i) and (ii) for each of said candidate vehicles; (iv) selecting one of said candidate vehicles based on said attributed values; and (v) allocating said selected candidate vehicle the next place in said sequence.

This invention relates to a method of sequencing vehicles. It hasparticular application for establishing the landing sequence ofaircraft.

A phenomenon known as ‘wake turbulence’ is caused by wake vortices,which form whenever an aircraft wing is producing lift. The pressuredifferential between the top and bottom surfaces of the wing triggersthe roll-up of the airflow aft of the wing resulting in swirling massesof air trailing downstream of the wing tips. The intensity or strengthof the vortices are primarily a function of the aircraft weight with thestrongest vortices being produced by heavy aircraft.

Flying into the vortices can cause imbalance in following aircraft(possibly causing the following aircraft to crash) especially if themass of the following aircraft is too small or the intensity of thevortices is too great. As a result, a delay between two successiveaircraft landings has to be maintained to avoid this potentiallyhazardous situation. This delay has to be extended proportionally to themass ratio of the leading and following aircraft.

Assuming that there are three categories of aircraft (“heavy”, “large”and “small”) and that the safe delay between them is an incrementalfunction of their relative size, FIG. 1 shows a table summarising thedelays (in time units, e.g. minutes) that must be maintained betweensuccessive landings. If all aircraft belonged to just one category thenthe delay would always be minimal. The delay would also be minimal ifthe arriving air traffic was grouped into three sets with all “small”aircraft landing first, followed by all “large” aircraft and followedfinally by all “heavy” aircraft. It is, of course, highly unlikely thattimetable requirements would allow the organisation of air traffic intosuch a perfectly ordered sequence. In fact, aircraft belonging to allthree categories follow each other at random and a problem facing airtraffic controllers is choosing the aircraft which should be allowed toland next.

Two systems currently used by air traffic controllers to sequenceincoming aircraft and to ensure landing aircraft are safely separatedare Traffic Management Advisor (TMA) and Final Approach Spacing Tool(FAST) both developed by the National Aeronautics and SpaceAdministration (NASA) Ames Research Centre, Moffett Field, Calif. 94035,USA.

Both these systems sequence incoming aircraft on a first come, firstserved (FCFS) basis where the first incoming aircraft to contact airtraffic control (ATC) (with a request to land) on entering the terminalarea (a term used to describe airspace in which air traffic controlservice is provided to aircraft arriving and departing an airfield) isallocated a landing slot first and placed at the start of the sequence.Subsequent, incoming aircraft are placed in the sequence in the order inwhich they enter the terminal area and contact ATC. Appropriate spacingis applied between sequenced aircraft to comply with safety constraints.It has been found, however, that sequencing aircraft on a FCFS basisleads to a less than optimal landing rate which leads to increaseddelays for arriving aircraft as they are forced to wait in the terminalarea (usually in a waiting/holding stack) to be allocated a landingslot. This in turn leads to a reduction in quality of service providedby airlines and also to a increase in fuel consumption for the waitingaircraft.

According to a first aspect of the present invention there is provided amethod of sequencing a plurality of candidate vehicles, wherein eachcandidate vehicle in said plurality of candidate vehicles is a candidateto be allocated the next place in a sequence, said method comprising thesteps of:

(i) receiving information pertaining to one of said candidate vehicles;

(ii) calculating a value to be attributed to said candidate vehicle onthe basis of said received information and information received from thecandidate vehicle most recently allocated a place in said sequence;

(iii) repeating steps (i) and (ii) for each of said candidate vehicles;

(iv) selecting one of said candidate vehicles based on said attributedvalues; and

(v) allocating said selected candidate vehicle the next place in saidsequence.

Preferably the plurality of candidate vehicles comprises a plurality ofcandidate aircraft and the sequence is the landing sequence. By usinginformation pertaining to candidate aircraft information from theaircraft most recently allocated a place in the sequence, a value can becalculated for each of the candidate aircraft and one of the candidateaircraft can be selected and allocated the next place in the sequence.The sequence of aircraft thus generated is more optimal than sequencesotherwise generated, for example on a “first come, first served” basis.

Preferably, said received information is received from the candidatevehicle to which said received information pertains. In this way, it ismore than likely that the received information will be up-to-date.

Preferably, said value is representative of the spacing that would haveto be maintained between the candidate vehicle and the candidate vehiclemost recently allocated a place in said sequence if said candidatevehicle were allocated the next place in the sequence. In this way, theaverage interval between successive vehicles is reduced.

Preferably, said value is representative of the delay that would beexperienced by said candidate vehicle if said candidate vehicle wereallocated the next place in the sequence. In this way, the average delayexperienced by the candidate vehicles is reduced.

According to a second aspect of the present invention, there is provideda method of operating a sequencing apparatus to sequence a plurality ofcandidate vehicles, wherein each candidate vehicle in said plurality ofcandidate vehicles is a candidate to be allocated the next place in asequence, said method comprising the steps of:

(i) receiving information pertaining to one of said candidate vehicles;

(ii) calculating a value to be attributed to said candidate vehicle onthe basis of said received information and information received from thecandidate vehicle most recently allocated a place in said sequence;

(iii) repeating steps (i) and (ii) for each of said candidate vehicles;

(iv) selecting one of said candidate vehicles based on said attributedvalues; and

(v) allocating said selected candidate vehicle the next place in saidsequence.

Preferably, said method further comprises the step of:

(vi) sending details of the next place in said sequence to said selectedcandidate vehicle.

According to a third aspect of the present invention there is providedsequencing apparatus arranged in operation to sequence a plurality ofcandidate vehicles, wherein each candidate vehicle in said plurality ofcandidate vehicles is a candidate to be allocated the next place in asequence, said data processing apparatus comprising:

-   -   receiving means for receiving information pertaining to one of        said candidate vehicles;    -   calculating means for calculating a value to be attributed to        said candidate vehicles on the basis of said received        information and information received from the candidate vehicle        most recently allocated a place in said sequence;    -   selecting means for selecting one of said candidate vehicles        based on said attributed values; and    -   allocating means for allocating said selected candidate vehicle        the next place in said sequence.

According to a fourth aspect of the present invention there is providedsequencing apparatus arranged in operation to sequence a plurality ofcandidate vehicles, wherein each candidate vehicle in said plurality ofcandidate vehicles is a candidate to be allocated the next place in asequence, said data processing apparatus comprising:

-   -   a receiver arranged in operation to receive information        pertaining to one of said candidate vehicles;    -   a calculator arranged in operation to calculate a value to be        attributed to said candidate vehicles on the basis of said        received information and information received from the candidate        vehicle most recently allocated a place in said sequence;    -   a selector arranged in operation to select one of said candidate        vehicles based on said attributed values; and    -   an allocator arranged in operation to allocate said selected        candidate vehicle the next place in said sequence.

According to a fifth aspect of the present invention there is provided adigital data carrier carrying a program of instructions executable byprocessing apparatus to perform the method steps as set out in the firstaspect of the present invention.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, wherein likereference numerals refer to like parts, and in which:

FIG. 1 shows a table summarising the delays that must be maintainedbetween successive landings of aircraft;

FIG. 2 illustrates aircraft approaching a destination airfield;

FIG. 3 illustrates a schematic view of the software used to implement anembodiment the present invention;

FIG. 4 is a flow diagram illustrating the first stages of an aircraftsequencing process;

FIG. 5 is a flow diagram illustrating the remaining stages of anaircraft sequencing process;

FIG. 6 is a flow diagram illustrating the calculation of a cost functionin accordance with an embodiment of the present invention;

FIG. 7 is a flow diagram illustrating the computation of a landing timeslot in accordance with an embodiment of the present invention.

FIG. 8 is a table showing the results of sequencing aircraft on a “firstcome, first served” basis;

FIG. 9 is a table showing the results of sequencing aircraft inaccordance with an embodiment of the present invention;

FIG. 10 is a graph showing a comparison in delays suffered by aircraftsequenced on a “first come, first served” basis and delays suffered byaircraft sequenced in accordance with an embodiment of the presentinvention.

In reference to FIG. 2, a plurality of aircraft 201 are shownapproaching a destination airfield within a terminal area under thecontrol of terminal area ATC 203. In order to request a landing timeslot at the destination airfield, each of the aircraft 201 must contactterminal area ATC 203 upon entering the terminal area. The aircraftarrive in the terminal area in an unpredictable fashion, i.e. in arandom order.

A computer 205 within terminal area ATC 203 operates under the controlof software executable to carry out an aircraft sequence selectingprocess. As will be understood by those skilled in the art, any or allof the software used to implement the invention can be contained onvarious transmission and/or storage media such as floppy disk, CD-ROM ormagnetic tape so that it can be loaded onto the computer or could bedownloaded over a computer network using a suitable transmission medium.

Referring to FIG. 3, the software loaded onto computer 205 operates byattributing and/or revising the priorities of entities (E₁, E₂, E₃, . .. , En) within a dynamic set 301. Associated with each entity (E_(n)) isa collection of real-time variables [x(E_(n)), y(E_(n))]. The softwarefurther includes a scheduler 303 which operates in accordance with anoptimisation algorithm in order to update the priority of the entitiesstored in the dynamic set 301 and move them to a static set 305. Eachentity represents a single aircraft arriving into the terminal area.Aircraft wait to be allocated a landing time slot in a waiting/holdingstack represented by the dynamic set 301. Using the optimisationalgorithm, ATC (represented by the scheduler 303) decides the order ofthe landing sequence which is represented by the static set 305.Examples of the real time variables associated with each entity are theflight identification number of the aircraft, the size of the aircraftand the estimated time of arrival (ETA) of the aircraft at itsdestination.

Two further real time variables, I_(n) and D_(n), are defined per entityfor use in the algorithm by the scheduler. I_(n) is the interval to theaircraft represented by the latest entity in the static set 305 shouldthe aircraft represented by entity E_(n) be allocated the next landingslot. (It will be remembered that this was described above in relationto FIG. 1.) D_(n) is the delay of the aircraft represented by entityE_(n) when compared with the aircraft's ETA should the aircraftrepresented by entity E_(n) be allocated the next landing slot.

The two variables, I_(n) and D_(n), are combined into a cost functionf(I,D) which represents the associated ‘cost’ of allocating the nextavailable landing time slot to the aircraft represented by the entityE_(n). The relative weights of the two variables, I_(n) and D_(n), inthe cost function are adjustable and are defined as the value of twoexponents, α and β. The cost function f(I_(n),D_(n)) is shown in full inequation [1] below: $\begin{matrix}{{f\left( {I_{n},D_{n}} \right)} = \frac{I_{n}^{\alpha}}{D_{n}^{\beta}}} & \lbrack 1\rbrack\end{matrix}$

The cost of selecting one entity from the dynamic set and transferringit to the static set (i.e. allocating the next available timeslot to anaircraft represented by entity E_(n)) is directly proportional to theinterval, I_(n) raised to the power α and inversely proportional to thedelay, D_(n) raised to the power β. A low interval and a high delay willdecrease the cost of selecting a particular entity and hence decreasethe cost of allocating a landing time slot to the represented aircraft.The longer an aircraft has already been waiting to be allocated a timeslot, the more likely it becomes that it is allocated the next availabletime slot. However, all things being equal (i.e. all aircraft havingsimilar delays), the aircraft with the shortest interval will beselected. This is best for maximising throughput of aircraft, reducingthe chance of a long queue of waiting aircraft and therefore benefitingboth the airfield and the aircraft.

Increasing α increases the weight of the interval I_(n) at the expenseof the delay D_(n). This typically results in minimal intervals betweensuccessive aircraft. On the other hand, increasing β increases theweight of the delay D_(n) at the expense of the interval I_(n) whichtypically results in reduced delays and hence reduced waiting times forincoming aircraft.

In preferred embodiments, the values of α and β are set to α=1.0 andβ=2.0. However, it is possible to modify the values of α and/or β toreflect changing priorities. Different aircraft may have differentpriorities due to, for example, an emergency situation on board theaircraft, the amount of fuel the aircraft is carrying, the totalduration of the aircraft's journey, the nature of the cargo and/or thepassengers onboard the aircraft etc. As a result, what is considered tobe an ‘acceptable delay’ may vary accordingly.

Moreover, other variables and/or parameters could be added to equation[1] to account for other factors not included in the preferredembodiment, for example, the intrinsic priority of the aircraft, thecurrent fuel consumption and/or fuel load of the aircraft, currentatmospheric conditions, weather forecast etc. This would only modify theoutput variable returned by the cost function which is used as adecision basis by the scheduler.

In preferred embodiments, the decision as to which entity should bemoved from the dynamic set to the static set and hence which aircraftshould be allocated the next available landing time slot is madedeterministically, that is, the entity with the lowest cost is moved.

Referring now to FIG. 4, on entering the terminal area, an approachingaircraft 201 contacts terminal area ATC 203 (step 401) via radiocommunication with a request for a landing time slot. This is assumed totake place anytime between ten and twenty minutes before the estimatedtime of arrival (ETA) of the aircraft at its destination. This initialcontact message contains information such as a flight identificationnumber of the aircraft, the size of the aircraft and the ETA of theaircraft. Upon receiving the contact message, terminal area ATC 203acknowledges the message by sending a message back to the requestingaircraft 201 (step 403) which includes an order to wait in thewaiting/holding stack. At the same time, an entity representing therequesting aircraft 201 is created by terminal area ATC 203 and added tothe dynamic set 301 (step 405). This is achieved, for example, byinputting the relevant information onto computer 205 via a keyboard (orother such input device) attached to the computer 205. In otherembodiments, the information could be entered automatically intocomputer 205 via a datalink established between the requesting aircraftand terminal area ATC 203. Associated with that entity are the real timevariables representing the information sent by the aircraft to terminalarea ATC 203. The process described above in relation to FIG. 4 isrepeated whenever an aircraft 201 enters the terminal area. Severalaircraft 201 may enter the terminal area every minute contactingterminal area ATC 203 with a request for a landing time slot. Thisresults in several entities being created and added to the dynamic set.

Referring to FIG. 5, the operation of the scheduler will now bedescribed in further detail. Firstly, a new session of the scheduler isinitialised (step 501). A new session is begun for each landing timeslot that is to be allocated by the scheduler. In preferred embodimentsthe scheduler is run once every minute although in other embodimentsmore or less sessions per minute may be more suitable.

The scheduler then extracts information (step 503) for the next entityrepresenting an aircraft that has contacted terminal area ATC 203. Theinformation extracted is that which the aircraft sent to terminal areaATC 203 in its initial contact message (FIG. 4, step 401). The schedulerthen checks (step 505) whether or not the entity currently beingprocessed has been waiting in the dynamic set for over a specifiedperiod of time, e.g. thirty minutes. (It will be realised that thiscorresponds to an aircraft waiting in the waiting/holding stack for morethan thirty minutes.) If this check yields a positive result thenterminal area ATC 203 contacts the aircraft represented by this entityin order to re-direct it to another airfield (step 507) and therepresentative entity is removed from the dynamic set. If the check isnegative then the scheduler continues to calculate the cost function forthis entity (step 509). The calculation of the cost function will bedescribed in more detail below.

The scheduler then checks (step 511) whether or not the cost functionjust calculated is the lowest so far calculated in this session. If itis the lowest so far calculated then this entity is temporarilyclassified as the best choice entity (step 513) until a time when thecost function of another entity is lower. Having calculated the costfunction for the first entity in the current session, the scheduler thenchecks (step 515) whether or not cost functions for all the entitiescurrently within the dynamic set have been calculated. If the result ofthis check is negative then steps 503 to 515 are repeated. If costfunctions have been calculated for all the entities currently within thedynamic set then the entity that ends up classified as the best choiceentity is moved from the dynamic set to the static set (step 517) andthe scheduler computes (step 518) the next available landing time slotto allocate to the aircraft represented by the best choice entity. Thecomputation of the landing time slot will be described in more detailbelow.

Having computed the landing time slot to be allocated to the aircraftrepresented by the best choice entity, the scheduler checks whether ornot the delay associated with that aircraft (i.e. the difference betweenits allocated landing time slot and its ETA) is longer than a specifiedtime period, e.g. sixty minutes. If the result of this check is positivethen terminal area ATC 203 contacts the aircraft in order to re-directit to another airfield (step 521) after which time a new session of thescheduler is started. If the result of the check is negative thenterminal area ATC 203 contacts the aircraft and informs it of itsallocated landing time slot (step 523) at which time a new session ofthe scheduler is started.

With reference to FIG. 6, the calculation of the cost function (carriedout in step 509) will now be explained in more detail. The schedulerfirst extracts information (step 601) from the last entity that wasmoved from the dynamic set to the static set. It will be realised thatthis entity represents the most recent aircraft to be allocated alanding time slot. The information extracted includes the size of themost recent aircraft and the landing time slot allocated to it. Usingthis information and the size of the aircraft represented by the entitycurrently being processed (which it will be remembered was extracted instep 503), the scheduler then computes (step 603) what the interval (I)between these two aircraft would have to be if the aircraft representedby the entity currently being processed were allocated the next landingtime slot. In the present embodiment, the intervals between successiveaircraft are those described above in relation to the table in FIG. 1,although otherwise defined intervals are also possible. The schedulercan then add this interval to the landing time slot allocated to themost recent aircraft to compute (step 605) a proposed landing time slotfor the aircraft represented by the entity currently being processed.The scheduler can then compute the delay (D) (step 607) that theaircraft represented by the entity currently being processed wouldsuffer if allocated this landing time slot by comparing it with theaircraft's ETA. Finally the scheduler can use the interval I and delay Dto compute the cost function (step 609) of the entity currently beingprocessed.

With reference to FIG. 7, the computation of the landing time slot(carried out in step 518) will now be described in more detail. Thescheduler first extracts information (step 701) from the last entitythat was moved from the dynamic set to the static set. It will berealised that this entity represents the most recent aircraft to beallocated a landing time slot. The information extracted includes thesize of the most recent aircraft and the landing time slot allocated toit. Using this information and the size of the aircraft represented bythe best choice entity extracted by the scheduler in step 703, thescheduler then computes (step 705) what the interval (I) between thesetwo aircraft has to be based on the intervals defined above in relationto the table in FIG. 1. Finally, the scheduler adds this interval to thelanding time slot allocated to the most recent aircraft to compute (step707) the landing time slot for the aircraft represented by the bestchoice entity.

It will be realised that in calculating the cst function for the bestchoice entity (in step 509), a proposed landing time slot for theaircraft represented by the best choice entity is calculated (in step605). Hence in alternative embodiments, this information could betemporarily stored by the computer 205 and used by terminal area ATC 203when it contacts the aircraft and informs it of its allocated landingtime a lot (in step 523).

FIG. 8 illustrates the landing sequence for the period 08:17 to 08:59made on a “first come, first served” basis. FIG. 9 illustrates thelanding sequence for the same period and for an identical trafficpattern (same aircraft, same order of arrival) computed in accordancewith the present invention.

The tables in both FIGS. 8 and 9 are sorted by “Landing Time” whichrefers to the time the aircraft lands at its destination. “Flight ID”refers to the flight identification number of the aircraft, “Cat.”refers to the size category of the aircraft, “ATC contact” refers to thetime that the aircraft sends its initial contact message to terminalarea ATC 203 on entering the terminal area, “ETA” refers to theaircraft's estimated time of arrival at its destination, “ATC allocate”refers to the time when terminal area ATC 203 contacts the aircraft withdetails of its allocated landing time slot and “Delay” refers to thedifference in time between the aircraft's ETA and its actual landingtime.

The shaded rows in the table 9 indicate aircraft that contacted terminalarea ATC 203 earlier than some of the preceding aircraft but wereallocated landing time slots later than these predecessors. (This seriesof events can occur when the landing sequence is decided on a “firstcome, first served” basis but only when an aircraft that contactsterminal area ATC 203 has a later ETA than some of the followingaircraft. This is indicated by the shaded rows in table 8.)

Referring to FIG. 8, thirty aircraft land in the forty-two minute periodbetween 08:17 and 08:59. The average interval between them is oneminute, twenty-five seconds and the average delay suffered by eachaircraft is eighteen minutes, forty-four seconds. Referring to FIG. 9,thirty-seven aircraft land in the same forty-two minute period. Theaverage interval between them is now only one minute, ten seconds andthe average delay suffered by each aircraft has fallen to fifteenminutes, sixteen seconds. This represents a 23.3% increase in capacityat the destination, a 18.5% reduction in the average delay suffered byarriving aircraft and a 17.4% reduction in the average interval betweensuccessive aircraft landings. This translates into a large improvementin quality of service for the airlines operating the aircraft, includinga substantial reduction in fuel consumption and an increase in revenuefor airfields due to the increase in capacity.

The graph in FIG. 10 summarises the comparison. It is a plot of thedelay suffered by aircraft against the time of day at which they land attheir destination. By noon, nearly all flights are delayed by at leastthirty minutes and the situation continues to deteriorate since in theabsence of any optimisation, the extra air traffic cannot be absorbedand the waiting/holding queue can only continue to grow. In contrast,the delay suffered by flights sequenced in accordance with the presentinvention remains fairly constant throughout the day. By the end of theday, three aircraft sequenced on a “first come, first served” basis hadto be re-routed to another destination because they suffered delaysexceeding the maximum allowed delay (one hour in this case). The averagedelay suffered by aircraft was above thirty minutes compared with lessthan ten minutes for aircraft sequenced in accordance with the presentinvention.

Although in the above described embodiment the decision as to whichentity should be moved from the dynamic set to the static set and hencewhich aircraft should be allocated the next available landing time slotis made deterministically, it is also possible to make the decisionprobabilistically on the basis of a function similar to:$C_{x} = \frac{f\left( {I_{x},D_{x}} \right)}{\sum\limits_{i = 1}^{N}{f\left( {I_{i},D_{i}} \right)}}$$P_{x} = \frac{1 - C_{x}}{{\sum\limits_{i = 1}^{N}1} - C_{i}}$where N is the number of entities currently waiting in the dynamic set,C_(x) is the relative cost of selecting entity x and P_(x) is theprobability that entity x is chosen.

Although the above embodiment was described in relation to the landingsequence of aircraft, it will be apparent that the present invention isjust as applicable to the sequencing of any vehicles in a situationwhere those vehicles disturb the environment behind them as theyproceed. One example of such a situation is ships/boats which leave awake behind them.

The present invention successfully optimises sequences of vehicles. Testresults suggest that sequencing aircraft about to land in accordancewith the present invention leads to an increase in capacity at airfields(since aircraft can land more often) and an improvement to the qualityof service provided by airlines operating those aircraft (since thedelays suffered by aircraft is reduced). These two objectives werepreviously thought to be incompatible.

1. A method of sequencing a plurality of candidate vehicles, whereineach candidate vehicle in said plurality of candidate vehicles is acandidate to be allocated the next place in a sequence, said methodcomprising the steps of: (i) receiving information pertaining to one ofsaid candidate vehicles; (ii) calculating a value to be attributed tosaid candidate vehicle on the basis of said received information andinformation received from the candidate vehicle most recently allocateda place in said sequence; (iii) repeating steps (i) and (ii) for each ofsaid candidate vehicles; (iv) selecting one of said candidate vehiclesbased on said attributed values; and (v) allocating said selectedcandidate vehicle the next place in said sequence.
 2. A method asclaimed in claim 1, wherein said vehicles are aircraft.
 3. A method asclaimed in claim 2, wherein said sequence is the landing sequence.
 4. Amethod as claimed in claim 1, wherein said received information isreceived from the candidate vehicle to which said received informationpertains.
 5. A method as claimed in claim 1, wherein said receivedinformation includes information relating to the size of the candidatevehicle to which said information pertains.
 6. A method as claimed inclaim 1, wherein said value is representative of the spacing that wouldhave to be maintained between the candidate vehicle and the candidatevehicle most recently allocated a place in said sequence if saidcandidate vehicle were allocated the next place in the sequence.
 7. Amethod as claimed in claim 1, wherein said value is representative ofthe delay that would be experienced by said candidate vehicle if saidcandidate vehicle was allocated the next place in the sequence.
 8. Amethod of operating a sequencing apparatus to sequence a plurality ofcandidate vehicles, wherein each candidate vehicle in said plurality ofcandidate vehicles is a candidate to be allocated the next place in asequence, said method comprising the steps of: (i) receiving informationpertaining to one of said candidate vehicles; (ii) calculating a valueto be attributed to said candidate vehicle on the basis of said receivedinformation and information received from the candidate vehicle mostrecently allocated a place in said sequence; (iii) repeating steps (i)and (ii) for each of said candidate vehicles; (iv) selecting one of saidcandidate vehicles based on said attributed values; and (v) allocatingsaid selected candidate vehicle the next place in said sequence.
 9. Amethod as claimed in claim 8 further comprising the step of: (vi)sending details of the next place in said sequence to said selectedcandidate vehicle.
 10. A method as claimed in claim 8, wherein saidvehicles are aircraft.
 11. A method as claimed in claim 10, wherein saidsequence is the landing sequence.
 12. Sequencing apparatus arranged inoperation to sequence a plurality of candidate vehicles, wherein eachcandidate vehicle in said plurality of candidate vehicles is a candidateto be allocated the next place in a sequence, said data processingapparatus comprising: receiving means for receiving informationpertaining to one of said candidate vehicles; calculating means forcalculating a value to be attributed to said candidate vehicles on thebasis of said received information and information received from thecandidate vehicle most recently allocated a place in said sequence;selecting means for selecting one of said candidate vehicles based onsaid attributed values; and allocating means for allocating saidselected candidate vehicle the next place in said sequence. 13.Sequencing apparatus arranged in operation to sequence a plurality ofcandidate vehicles, wherein each candidate vehicle in said plurality ofcandidate vehicles is a candidate to be allocated the next place in asequence, said data processing apparatus comprising: a receiver arrangedin operation to receive information pertaining to one of said candidatevehicles; a calculator arranged in operation to calculate a value to beattributed to said candidate vehicles on the basis of said receivedinformation and information received from the candidate vehicle mostrecently allocated a place in said sequence; a selector arranged inoperation to select one of said candidate vehicles based on saidattributed values; and an allocator arranged in operation to allocatesaid selected candidate vehicle the next place in said sequence. 14.Sequencing apparatus according to claim 13, wherein said vehicles areaircraft.1
 15. Sequencing apparatus according to claim 14, wherein saidsequence is the landing sequence.
 16. A digital data carrier carrying aprogram of instructions executable by processing apparatus to performthe method steps as set out in claim 1.